Cylindrical rotary valves have been utilized in hydraulic devices for years. As long as the application of pressure to such valves is constant and known, a designer can compensate for any pressure induced dimensional changes with relative ease. However, if the pressure is not constant, as in a bidirectional variable speed motor, the pressure induced dimensional changes can create significant loss in the volumetric efficiency, mostly due to increased leakage. This can reek havoc with the design.